FIELD OF THE INVENTION
Plasma-generating reactors have been used extensively in processes for fabricating integrated circuit or microelectromechanical (MEM) devices from a substrate such as a silicon wafer. One particularly useful reactor is the inductively-coupled plasma-generating (ICP) reactor, which inductively (and to some extent capacitively) couples radio frequency (RF) power into a gas contained within the reactor to generate a plasma. The plasma contains species such as ions, free radicals, and excited atoms and molecules that may be used to process the substrate and ultimately produce integrated circuit or MEM devices.
An ICP reactor may be used to carry out a variety of processes to fabricate integrated circuit devices from a semiconductor substrate, including anisotropic and isotropic etch and chemical vapor deposition (CVD). For anisotropic etch, an ICP reactor may be used to produce a plasma with a high ion density. Generally, a low pressure and high RF power are used which favor the production of ions. The ions are accelerated perpendicularly toward the surface of the substrate by an electric field which is typically induced by an RF bias on the wafer. The ions bombard the substrate and physically and/or chemically etch the substrate and any materials deposited thereon, such as polysilicon (poly), silica (SiO.sub.2, silicon oxide, or oxide), silicon nitride (Si.sub.3 N.sub.4 or nitride), photoresist (resist), or metal deposited on the substrate. Such anisotropic etch processes are useful for forming integrated circuit features having substantially vertical sidewalls.
ICP reactors are also useful for producing reactive species for isotropic etching, particularly for stripping photoresist from the surface of a semiconductor substrate. Sufficient energy is coupled into the gas in the plasma generation chamber to form a plasma containing a high density of atomic and molecular free radicals that chemically react with the polymeric photoresist to facilitate its removal. For example, a plasma may be used to dissociate oxygen gas into atomic oxygen that reacts with polymeric photoresist to form CO and CO.sub.2, which evolve and are carried away in the process gas in the reactor. In such processes, in contrast to anisotropic etch, it is often desirable to reduce or eliminate ion bombardment which may damage the surface of the substrate.
ICP reactors are also useful for CVD of a material onto the surface of a substrate. For many CVD processes, the process is enhanced by ion bombardment and may be carried out at lower temperatures with higher deposition rates by exposing the substrate directly to the plasma (plasma enhanced CVD). In CVD, sufficient energy is coupled into the gas in the plasma generation chamber to form a plasma containing a high density of atomic and molecular free radicals and energetic species that interact with the surface of the substrate to form a deposited layer. For example, silane (SiH.sub.4) releases hydrogen and can be used to deposit a layer of polysilicon onto a substrate. In addition, silane or TEOS can be added to an oxygen plasma to deposit a layer of silicon dioxide on a substrate, which can be used as an etch mask during reactive-ion etching or as an insulating layer in circuit devices.
In each of the above processes, processing uniformity is a critical factor in determining integrated circuit quality, yield, and production rate. Uniform etching, stripping, or chemical deposition over the surface of a wafer assures that structures fabricated at the center of the wafer's surface have essentially the same dimensions as structures fabricated near the edge of the wafer. Thus the yield of chips from a wafer depends at least in part on the etch, strip, or deposition uniformity across the wafer's surface. Higher yield also contributes to a higher production rate.
Processing uniformity may be affected by the density and distribution of reactive species in the plasma and by the plasma potential across the wafer's surface. Processing may occur at higher rates in areas of the wafer surface where there is a higher density of reactive species. Further, for ion enhanced processes, any variance in the plasma potential across the wafer's surface will cause a corresponding variance in ion bombardment energies which may, for example, lead to nonuniform ion etch or ion enhanced deposition.
A number of different inductively-coupled reactor configurations have been used to produce plasmas for wafer processing. Typically, a cylindrical reactor chamber surrounded by a helical induction coil is used for plasma processing, although hemispheric reactor chambers (see U.S. Pat. Nos. 5,346,578 and 5,405,480) and reactors with planar coils in a "pancake" configuration (see U.S. Pat. Nos. 5,280,154 and 4,948,458) have been used as well. In typical conventional reactors, a plasma of acceptable uniformity can be produced provided that the diameter of the substrate and, consequently the reactor chamber, is not too large.
In an effort to increase chip production rates, however, integrated circuit manufacturers have moved from small-diameter substrates to substrates of ever-increasing diameter. At one time, 100 millimeter (mm) silicon wafers were the norm. These wafers were subsequently replaced by 150 mm and then 200 mm wafers. 300 mm wafers have been produced and will undoubtedly become the standard wafer for high-volume and high complexity computer chips in the near future. In time, it is expected that even larger wafers will be developed.
With larger diameter substrates, it becomes difficult to produce a plasma with highly uniform properties in a conventional cylindrical reactor chamber. For ion enhanced processes, the flux of ions across the wafer surface may become nonuniform. FIG. 1 illustrates a typical cylindrical ICP reactor, generally indicated at 100. In reactor 100, gas is provided to the reactor chamber 102 through an inlet 104. A helical induction coil 106 surrounds the chamber and inductively couples power into the gas in the reactor chamber to produce a plasma. Ions or neutral activated species then flow to a wafer surface 108 for processing. In addition, an RF bias may be applied to the wafer to accelerate ions toward the wafer surface for ion enhanced processing.
The dashed line 110 in FIG. 1 represents a stagnation surface for a plasma produced in the reactor of FIG. 1. The stagnation surface is the surface of maximum DC plasma potential. Ions inside the stagnation surface tend to fall to the wafer surface for processing, while ions outside the stagnation surface tend to fall to the walls of the reactor chamber. A higher percentage of ions near the edges of the wafer fall to the walls than near the center of the wafer as illustrated by the curved stagnation surface 110 in FIG. 1. This is a result of the proximity of the walls to the edges of the wafer and is also a function of the ion production rate in the reactor volume. In large diameter reactor chambers, the difference in the ion flux between the edges and the center of the wafer may be significant and lead to nonuniform processing. Even in non-ion enhanced processes, such as isotropic etch, nonuniform production of reactive species across a large diameter wafer surface may lead to nonuniform processing.
Thus, as larger diameter wafers are processed, problems are expected to be encountered with conventional inductively-coupled plasma reactor configurations. Moreover, integrated circuit features are expected to decrease in size, requiring increased processing uniformity.
What is needed is a plasma reactor with enhanced control over the plasma characteristics in the center of the chamber while allowing large diameter wafers to be processed. Preferably such a plasma reactor can be used to provides a uniform plasma potential and/or species concentration across the surface of a substrate for etching, stripping or chemical vapor-deposition and can be used to process smaller wafers such as 100 mm, 150 mm, and 200 mm wafers as well as 300 mm or larger wafers. In addition, for non-ion enhanced processes, such as photoresist strip, it is desirable to provide a reactor configuration that both enhances the uniform production of reactive species and provides a plasma generation volume that can be used to isolate the plasma from the wafer surface to reduce ion drive-in.